I have a cheaply built wooden cottage in an arid climate with occasional rain during the winter.

The roof consists of planks with tarboard nailed on top. The planks appear to be in poor condition from the general rough climate, and rain damage:

they sound hollow and the nails that hold the tarboard down are starting to come out:

Also, the tarboard is coming off on the sides:

My gut feeling is that basically the entire roof should be replaced. That is super expensive here, though, because any wood is imported from elsewhere.

Apart from the obvious things like putting the tarboard back where it's coming off, affixing it with ridges, and hammering in new nails, is there anything else I can do to make the roof last a couple more years, and fix any leaks that would cause rain damage inside?